Temperature-Dependent Optical Properties of Monocrystalline CaF₂, BaF₂, and MgF₂

The alkaline earth metal difluorides are critical optical components for applications in non-contact temperature sensors, thermal imaging, and infrared spectroscopy due to their characteristically low refractive index and wide optical transparency spanning the ultraviolet to mid-infrared. Despite their technological importance, a systematic investigation into the temperature dependence of their optical properties is lacking. In this study, spectroscopic ellipsometry was used to obtain the refractive index of monocrystalline CaF₂, BaF₂, and MgF₂ for wavelengths between 220 nm and 1700 nm, and for temperatures between 21 °C and 368 °C. The refractive index of CaF₂ and BaF₂ was observed to decrease linearly with increasing temperature, which can be largely attributed to a reduction in the mass density due to thermal expansion. In contrast, the refractive index of MgF₂ was found to vary nonlinearly with temperature, which suggests competing effects from the material’s electronic polarizability. The temperature-dependent refractive index data reported here provide a finely-resolved mapping of the thermo-optic coefficient for these three materials, which could inform the development of optical devices operating at elevated or unsteady temperatures.